Stimulation of carbonic anhydrase activity and phosphorylation in primary astroglial cultures by norepinephrine

Phosphorylation increases the catalytic activity of rainbow trout gill cytosolic carbonic anhydrase

Cytoplasmic carbonic anhydrase (CAc) in the gill of teleost fish contributes to ionic regulation and acid–base balance by catalyzing the reversible reaction of CO2 and water, CO2 + H2O ↔ H(+) + HCO3(-). Regulation of CAc abundance and activity therefore is expected to fine-tune responses to ionic or acid–base challenges. The present study investigated the potential for gill CAc of rainbow trout, Oncorhynchus mykiss (tCAc), to undergo reversible phosphorylation. The activity of tCAc was approximately doubled by phosphorylation achieved through in vitro stimulation of endogenous protein kinases; kinase stimulation doubled phospho-threonine content from that observed in tCAc isolated under conditions where both kinases and protein phosphatases were inhibited. In vitro incubation to preferentially stimulate specific kinases implicated protein kinase G (PKG) in mediating the increase in tCAc activity. The kinetic parameters of turnover number (k cat) and substrate affinity (K m) were similarly affected by stimulation of either kinase or phosphatase action. However, phosphorylation via kinase stimulation significantly increased the efficiency of tCAc (V max /K m), and this factor may have contributed to the elevation of tCAc activity. In addition, phosphorylation of tCAc by kinase stimulation significantly increased the inhibition constant (K i) for acetazolamide. These results demonstrate that tCAc is subject to reversible phosphorylation; future work should focus on identifying the physiological situation(s) in which phosphorylation of trout branchial CAc occurs.

Early cytokinin response proteins and phosphoproteins of Arabidopsis thaliana identified by proteome and phosphoproteome profiling

Cytokinins are plant hormones involved in regulation of diverse developmental and physiological processes in plants whose molecular mechanisms of action are being intensely researched. However, most rapid responses to cytokinin signals at the proteomic and phosphoproteomic levels are unknown. Early cytokinin responses were investigated through proteome-wide expression profiling based on image and mass spectrometric analysis of two-dimensionally separated proteins and phosphoproteins. The effects of 15 min treatments of 7-day-old Arabidopsis thaliana seedlings with four main cytokinins representing hydroxyisopentenyl, isopentenyl, aromatic, and urea-derived type cytokinins were compared to help elucidate their common and specific function(s) in regulating plant development. In proteome and phosphoproteome maps, significant differences were reproducibly observed for 53 and 31 protein spots, respectively. In these spots, 96 proteins were identified by matrix-assisted laser desorption/ionization time-of-flight/time-of-flight mass spectrometry (MALDI-TOF/TOF MS), providing a snapshot of early links in cytokinin-regulated signalling circuits and cellular processes, including light signalling and photosynthesis, nitrogen metabolism, the CLAVATA pathway, and protein and gene expression regulation, in accordance with previously described cytokinin functions. Furthermore, they indicate novel links between temperature and cytokinin signalling, and an involvement of calcium ions in cytokinin signalling. Most of the differentially regulated proteins and phosphoproteins are located in chloroplasts, suggesting an as yet uncharacterized direct signalling chain responsible for cytokinin action in chloroplasts. Finally, first insights into the degree of specificity of cytokinin receptors on phosphoproteomic effects were obtained from analyses of cytokinin action in a set of cytokinin receptor double mutants.

pH transients evoked by excitatory synaptic transmission are increased by inhibition of extracellular carbonic anhydrase

Excitatory synaptic transmission has been associated with a rapid alkalinization of the brain extracellular space. These pH shifts are markedly increased by acetazolamide, an inhibitor of carbonic anhydrase. Although this effect can be readily explained by inhibition of extracellular carbonic anhydrase, this enzyme has been considered strictly intracellular in the central nervous system. To determine whether these alkaline shifts are regulated by extracellular carbonic anhydrase, we studied the effects of a membrane impermeant, dextran-bound inhibitor of this enzyme. Extracellular alkaline transients, measured with pH-sensitive microelectrodes, were generated in the CA1 region of rat hippocampal slices by repetitive electrical stimulation of Schaeffer collateral fibers or by local ejection of glutamate. More direct alkalinizations were elicited by focal ejection of NaOH in the vicinity of a pH microelectrode. These pH transients were reversibly enhanced by addition of the dextran-bound inhibitor. We conclude that there is significant carbonic anhydrase activity in the extracellular space of the brain. We postulate that this enzyme functions in the regulation and modulation of extracellular pH transients associated with neuronal activity.

Glial contribution to seizure: carbonic anhydrase activity in epileptic mammalian brain

The activity of carbonic anhydrase (CA), a glial enzyme, was measured in the epileptic cortex of audiogenic DBA/2 mice and of cats with a freeze lesion. In mice, the activity increased with age from birth to 24 days, but were always higher in audiogenic mice than in normal C57/BL mice, reflecting species differences. The difference between the two strains increased sharply from 25 to 40 days of age, after the period of maximal audiogenic susceptibility. Acetazolamide, a CA-specific inhibitor, greatly decreased the seizure severity score of DBA/2 mice after a single intraperitoneal (i.p.) administration (150 mg/kg). After 24 days of age, when CA activities were high, the effect of acetazolamide was less important, suggesting that the increased cortical CA activity might reflect a protective mechanism. In cats with a freeze lesion, no significant changes in CA activities were observed in the actively discharging primary and secondary foci as compared with the nonepileptogenic perifocal cortex and the control cortex of sham-operated animals. The results indicate that the cortex of genetically susceptible audiogenic mice has an increased CA activity. The hypothesis of an adaptive glial mechanism, relating to the age-dependent decrease of seizure susceptibility in DBA/2 mice, is postulated.

Induction of c-fos and TIS genes in cultured rat astrocytes by neurotransmitters

The interaction of neurotransmitters with their specific receptors initiates a cascade of intracellular biochemical events which lead to induction of specific genes. Included in this cascade is the rapid and transient induction of a family of primary early response genes we term TIS genes (Lim et al.: Oncogene 1: 263-270, 1987). Expression of six TIS gene, including c-fos, was examined in secondary cultures of rat neocortical astrocytes exposed to muscarinic and adrenergic agonists and antagonists to study the early genomic responses which accompany neurotransmitter-induced alteration of glial morphology and physiology. Carbachol induced accumulation of mRNA for c-fos and the other TIS genes. Carbachol-mediated induction of TIS mRNA expression was sensitive to atropine blockade and was potentiated by lithium. Norepinephrine (NE), isoproterenol, or phenylephrine also induced TIS mRNA accumulation. In order to determine which second-messenger pathways mediate NE induction of TIS gene expression, the influences of the beta(B) antagonist propranolol (PR), the alpha I(AI) antagonist prazosin (PZ), and the alpha 2(A2) antagonist yohimbine (YB) were examined. The induction of TIS1 mRNA by NE was partially blocked by PR or PZ alone, and completely abolished by both antagonists in combination. YB had no effect on TIS1 mRNA expression. These results suggest that NE induces TIS1 mRNA through both B- and A 1-adrenergic, but not A2, pathways. The lack of effect of inhibitors of phospholipase A2 and cyclooxygenase suggests that the A1 component is mediated through a protein kinase C pathway. The induction of transient gene expression by neurotransmitters may mediate the secondary genomic responses and phenotypic changes occurring in astrocytes in response to alterations in neuronal neurotransmitter release.

Defective myelination in the optic nerve of the Browman-Wyse (BW) mutant rat

The Browman-Wyse (BW) rat displays a spectrum of ocular abnormalities which include myelination by Schwann cells of retinal ganglion cell (RGC) axons within the retina. Immunohistochemical and ultrastructural studies of the optic nerves of adult BW rats (30-60 days of age) with myelinated intraretinal axons were performed. Although individual nerves displayed considerable morphological variability, all were characterized by an initial dysmyelinated proximal segment which was separated from a normally myelinated distal segment by a transitional junctional zone. The proximal segment contained axons which were predominantly unmyelinated: where myelination occurred, almost all sheaths were Po-positive, proteolipid protein-negative, and the myelinating cell was a Schwann cell. In the distal segment the distribution of myelinated axons appeared to be normal, sheaths were PLP+, and the myelinating cell was an oligodendrocyte. Within the proximal segment, axons that were myelinated by Schwann cells were isolated by a basal lamina and expanded extracellular spaces from the bulk of other RGC axons within the optic nerve. Few carbonic anhydrase (CAII)+ or GalC+ oligodendrocytes were seen in proximal segments that contained Schwann cells: anti-CAII antibody stained atypical cells within the proximal segments which did not resemble CAII+ oligodendrocytes in the distal segment, and which were probably GalC-. Astrocytes appeared normal throughout the length of the nerve, and there was no morphological specialization at the junctional zone similar to that at the lamina cribrosa. The possible source(s) of the intraneural Schwann cells, and the pathogenetic mechanisms underlying the aberrant myelination of RGC axons within the BW optic nerve are discussed.

Mechanisms of tolerance to the anticonvulsant effects of acetazolamide in mice: relation to the activity and amount of carbonic anhydrase in brain

The mechanism by which tolerance develops to the anticonvulsant effects of acetazolamide (AZM) was investigated in Swiss-Webster mice. The effects of single and six daily doses of 40 mg or 200 mg/kg AZM on electroshock seizure threshold (EST), maximal electroshock (MES) seizure pattern, and on the activity and total amount of carbonic anhydrase II (CAII) in various subcellular fractions (cytosol, microsomes, and myelin) of cerebral cortex, cerebellum, and brainstem were assessed. The activity of CAII was measured by microassay, and the total amount was measured by immunoassay methods developed in this laboratory. From the activity (units per microgram of protein) and total amount (nanograms per microgram protein) data, the specific activity (units per nanogram CAII) of the enzyme was calculated. With multiple doses, tolerance developed to both elevation of the EST and modification of the MES pattern noted with single doses of AZM. Accompanying the development of tolerance to the anticonvulsant effects of AZM was an increase in both the activity and specific activity of CAII in the various subcellular fractions and areas of the brain. The effects were dose dependent. Tolerance to the EST-elevating effects of AZM correlated with increases in the activity, total amount, and specific activity of CAII in the myelin fraction of the cerebral cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Carbonic anhydrase-II messenger RNA in neurons and glia of chick brain: mapping by in situ hybridization

The enzyme carbonic anhydrase is widespread in brain tissue. In rodent brains it has been reported to be exclusively in oligodendroglia but there has been some debate about the generality of this finding. To investigate the cellular distribution of carbonic anhydrase by an independent technique, we have examined the chick brain by in situ hybridization to detect mRNA from the carbonic anhydrase-II gene, using as controls the actin and vimentin genes. The most intense carbonic anhydrase-II hybridization is to the choroid plexus, to the Bergmann glia of the cerebellum, and to the Müller cells in the retina. Elsewhere, some brain regions are negative while others show many individual strongly positive cells; carbonic anhydrase-II mRNA is particularly abundant in some parts of the hyperstriatum, tectum and thalamus. Some of the larger labelled cells are identifiable as neurons. By histochemistry, we confirm the presence of the carbonic anhydrase enzyme in choroid plexus and Bergmann glia, but the enzyme is also present in blood vessel walls where there is no carbonic anhydrase-II mRNA; this may be a different isozyme. During embryogenesis, carbonic anhydrase-II mRNA appears in the retina as early as two days of incubation, but does not appear in the brain until much later.

Beta-adrenergic receptors of cerebellar astrocytes in culture: intact cells versus membrane preparation

Experiments were carried out to assess: the influence of culture conditions on the expression of beta-adrenergic receptors in intact glial cells from the central nervous system; and the extent to which quantitation of receptor sites in membrane preparations reflects the receptor population of the whole cells they are derived from. Cerebellar astrocytes were chosen for this study since essentially one receptor subtype, the beta 2 one, is present in adult cerebellum. Intact, attached cerebellar astrocytes exhibit only one class of binding sites for the beta-adrenergic antagonist, [3H]CGP 12177. Replating of the astrocytes after a few days of culture in vitro induces an up-regulation of the receptors. This effect is particularly important when astrocytes are maintained for 6 days in the presence of horse serum, a condition that favors cellular differentiation. Only 30-50% of the beta-adrenergic receptors of the intact cells can be detected on membrane preparations. When membranes are prepared from astrocytes grown either in the presence of horse serum or under chemically controlled medium (i.e. under differentiation promoting conditions) two classes of binding sites for [125I](-)-iodocyanopindolol are revealed. Several hypotheses, mainly related to the morphology of the cells, may provide an explanation for such differences. Studies of the pharmacological specificity of receptors of membrane fractions show that cerebellar astrocytes cultured in vitro exhibit both beta 1 and beta 2 receptor subtypes. The beta 1 subtype receptors are slightly more abundant when astrocytes are grown in fetal calf serum (FCS), a condition under which they exhibit a polygonal, poorly differentiated morphology. When culture conditions favor cellular differentiation, more receptors of the beta 2 subtype are seen, which can be related to what is observed in the adult in vivo where the astrocytes exhibit a differentiated morphology.

Desensitization of beta-receptors on primary astrocyte cultures by norepinephrine but not by tricyclic antidepressants

Primary astrocyte cultures from neonatal rat cerebral hemispheres were treated chronically for up to 3 weeks with the tricyclic antidepressants amitryptyline (AMT) or desipramine (DMI), or acutely with AMT and DMI added at the same time as the agonist, norepinephrine (NE). AMT and DMI were added at concentrations from 10(-9) to 10(-5) M. Both types of treatment did not decrease the increase in cyclic AMP (cAMP) content of these cells in response to a 10 min exposure to 10(-5) M NE. Chronic exposure to the antidepressants also did not affect stimulation of cAMP by isoproterenol (iso) in both rat and mouse primary astrocyte cultures. In contrast to the lack of effect of the tricyclic antidepressants pretreatment of the cultures with 10(-5) M NE resulted in total inhibition of the cAMP response after 2 h, with a 50% decrease occurring in about 45 min. This is similar to the agonist-induced desensitization of the beta-receptor-adenylate cyclase system seen in many other cells. This effect could, in part, be a direct response to increased intracellular cAMP since pretreatment with 0.25 and 1.0 mM N6-2'-O-dibutyryl cAMP (DBcAMP) also resulted in total inhibition of the cAMP response after 4 h. Receptor labelling experiments using [125I]cyanopindolol showed no decreases in apparent binding sites up to 3 h after exposure to 10(-5) M norepinephrine, suggesting that the rapid desensitization of the cAMP response was primarily due to an uncoupling of the receptor from the adenyl cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of protein synthesis in mitochondria, synaptosomes, and intact brain cells

Qualitative aspects of protein synthesis in organelles and intact cultured cells of brain origin were compared to clarify the distinction between synaptosomal and mitochondrial protein synthesis. Brain mitochondria and synaptosomes were isolated either on a traditional Ficoll-sucrose gradient or by a new Percoll gradient procedure, and were incubated in an amino acid incorporation system containing [35S]methionine, then electrophoresed on gradient slab gels. Autoradiography of the gels revealed that in the presence of cycloheximide both mitochondria and synaptosomes synthesized at least 17 proteins in the 6,000-50,000 MW range, and that incubation with chloramphenicol reduced or eliminated these bands. With minor variation these patterns in the low-molecular-weight region also resembled patterns obtained from cycloheximide-inhibited rat liver mitochondria and intact brain cells (cultured glia, glioma, and neuroblastoma). In the higher molecular weight region of the gels (greater than 50,000) banding patterns were more complex and tended to differ between organelles and intact cells. These polypeptides probably reflect nonmitochondrial protein synthesis, and their variable response to inhibitors may account for confusion in the literature with regard to the effects of inhibitors of protein synthesis in brain mitochondria and synaptosomes.

Effects of acute and chronic phenytoin on the electrolyte content and the activities of Na+, K+-, Ca2+, Mg2+-, and HCO3- -ATPases and carbonic anhydrase of neonatal and adult rat cerebral cortex

Various parameters of anion and cation transport were measured in the cerebral cortex of neonatal (3-day-old) and adult rats following acute and chronic treatment with phenytoin (PHT). Acutely, PHT significantly inhibited the enzyme Na+, K+-ATPase in both neonatal and adult rats. This effect was accompanied by a significant increase in cerebral cortical Na+ content and a decrease in K+ content only in neonatal animals. Chronic treatment (two and four times a day for 7 days) of adult rats with PHT significantly reduced Na+ content without affecting whole homogenate Na+, K+-ATPase activity. The activity of this enzyme was markedly increased in the myelin- (glial product) and slightly decreased in the synaptosomal- (neuronal) fractions following chronic (four times a day for 7 days) PHT treatment. These results suggest that PHT differentially affects the two forms (neuronal and glial) of the enzyme Na+, K+-ATPase. The possible relevance of this hypothesis in relationship to the anticonvulsant and excitatory properties of PHT is discussed. Chronic (two and four times a day for 7 days) PHT treatment increased both DNA content and activity of the glial marker enzyme carbonic anhydrase. Activity of the mitochondrial enzyme HCO3- -ATPase was also increased following chronic PHT treatment. These two enzymes are intimately involved in the regulation of HCO3- -Cl- transport across glial cell and mitochondrial membranes, and these results suggest that PHT is able to affect beneficially glial regulatory processes. The ability to enhance glial regulation of anions and cations in extracellular fluid provides new and important insights into the mechanism of the anticonvulsant action of PHT.

Effects of acetazolamide and electrical stimulation on cerebral oxidative metabolism as indicated by the cytochrome oxidase redox state

The intravenous administration of acetazolamide to rats caused a prompt oxidation of cytochrome a,a3 that was associated with an increase in the rate at which this cytochrome underwent additional oxidation and reductive recovery after electrical stimulation of the cerebral cortex. These effects were not observed in animals made hypercapnic after ventilation with 5% CO2. The speed with which these and other metabolic and physiological events occur, after administering the drug, suggests that acetazolamide exerts its effects by complex mechanisms and that the site of action may be in the region of the blood-brain barrier, an area rich in carbonic anhydrase, and noradrenergic innervation.

Swelling and ion uptake in cat cerebrocortical slices: control by neurotransmitters and ion transport mechanisms

Cat cerebrocortical slices incubating in medium containing normal K+ concentrations were exposed to a number of different transmitters. Norepinephrine, histamine and adenosine or 2-chloroadenosine caused increased swelling of the slices associated with an increased Na+ and Cl- content. These effects were seen only when both Cl- and HCO3- were present in the medium, and were inhibited by a number of anion transport inhibitors. These characteristics were identical to those of the HCO3(-)-dependent component of the swelling induced by high K+ levels in the medium. Other transmitters, namely 5-hydroxytryptamine, dopamine, and gamma-amino butyric acid, were ineffective. The effects of norepinephrine, histamine and 2-chloroadenosine were antagonised by propranolol and phentolamine, chlorpheniramine and diphenhydramine, and theophylline respectively. These antagonists also inhibited HCO3(-)-dependent, K+-stimulated swelling. The transmitters which induced swelling also stimulated the carbonic anhydrase activity of cerebrocortical slices. We conclude from these data that the HCO3(-)-dependent component of K+-stimulated swelling may be due to K+-stimulated release of transmitters. Furthermore, the fact that the transmitters which induce swelling have also been reported to be most effective in increasing cAMP content in both brain slices or cultured astrocytes is consistent with the swelling response being mediated via cAMP-induced changes and being predominantly localized to astrocytes.

Carbonic anhydrase, 5'-nucleotidase, and 2',3'-cyclic nucleotide-3'-phosphodiesterase activities in oligodendrocytes, astrocytes, and neurons isolated from the brains of developing rats

The activities of three myelin-associated enzymes, carbonic anhydrase, 5'-nucleotidase, and 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNP), were measured in oligodendrocytes, neurons, and astrocytes isolated from the brain of rats 10, 20, 60, and 120 days old. The carbonic anhydrase specific activity in oligodendrocytes was three- to fivefold higher than that in brain homogenates at each age, and, at all the ages, low activities of this enzyme were measured in neurons and astrocytes. The oligodendrocytes and astrocytes from the brains of rats at all ages had higher activities of the membrane-bound enzyme 5'-nucleotidase than was observed in neurons. In oligodendrocytes from 10- and 20-day-old rats, the 5'-nucleotidase activity was two-to threefold the activity in the homogenates (i.e., relative specific activity = 2.0-3.0), and the relative specific activity of this enzyme in the oligodendrocytes declined to less than 1.0 at the later ages, concomitant with the accumulation of 5'-nucleotidase in myelin. The CNP activity was always higher in oligodendrocytes than in neurons, but not appreciably different from that in astrocytes from 20 days of age onward. The relative specific activity of CNP was highest in the oligodendrocytes from 10-day-old rats but was lower, at all ages, than we had observed in bovine oligodendrocytes. These enzyme activities in oligodendroglia are quite different in amount and developmental pattern from those reported previously for myelin.

Immunocytochemical and biochemical analysis of carbonic anhydrase in primary astrocyte cultures from rat brain

Carbonic anhydrase (CA) was studied in primary monolayer cultures from neonatal rat cerebral hemispheres with both immunocytochemical and biochemical techniques. In such cultures, which consist predominantly of astrocytes, immunocytochemical staining for CA using antibody raised against the type II enzyme from rat erythrocytes resulted in positive staining of the flat, glial fibrillary acidic protein-positive, astrocytic monolayer. Smaller, process-bearing, round cells that grew on top of the astrocytes stained intensely for CA. We estimated that these cells represented 1% or less of the total cells in the cultures, and they have been identified by others as oligodendrocytes. The intensity of the staining of astrocytes for CA could be increased to that observed in oligodendrocytes when the astrocytes were made to round up and form processes by treatment with 2',3'-dibutyryl cyclic AMP. Enzymatic assays showed that CA activity of the cultures after 3 weeks of growth was 2.5- to 5-fold less than that found for cerebral homogenates from perfused 3-week-old rat brains. However, both activities were totally inhibited by acetazolamide with an I50 of 10(-8) M, confirming that both rat brain and the astrocyte cultures possess the high-activity type II enzyme. CA-II activity was unaffected by treatment of the cultures with a method reported to remove oligodendrocytes. Thus, the immunocytochemical and biochemical studies reported here demonstrate that astroglial cells in primary cultures from neonatal rat brain contain CA-II.

The phosphorylation of proteins in hippocampal slices: effects of noradrenaline and of pretreatment with kainic acid

Hippocampal slices were incubated in the presence of [32P]P1, and protein phosphorylation was examined by means of sodium dodecyl sulfate-gel electrophoresis. Incubation for at least 30 min with 300 muCi of [32P)P1/brain slice gave rise to the phosphorylation of 8-10 protein bands. Most of these bands showed enhanced phosphorylation in response to noradrenaline. The basal phosphorylation of kainic acid-pretreated hippocampal slices was enhanced two- to threefold compared with controls. There was also an additional increase in kainic acid-pretreatment slices in the response to noradrenaline. 8-Br-Cyclic AMP and phosphodiesterase inhibitors, such as papaverine or isobutylmethylxanthine, had no effect on the phosphorylation patterns.